Tiny sap thieves
319 words over 28 numbered lines, then 6 parts worth 12 marks (Modular Unit 2 (4WBI2) style, 12–13 marks). Read the article, answer each part, then open its mark scheme and set your mark.
Illustrative article written for Scholar Lab, not a Pearson text. The biology is real; the studies, people and figures are invented or rounded for practice unless a source is named.
Tiny sap thieves
In early summer, the young shoots of a rose bush can be covered with
small green insects called aphids. Each aphid pushes its needle-like
mouthparts into the stem until the tip reaches a single phloem tube.
The sap inside the phloem is under pressure, so it flows into the
aphid without the insect having to suck.
Phloem sap is rich in sucrose and amino acids. Sucrose is made from
the glucose produced by photosynthesis in the leaves, and it is
carried in the phloem to parts of the plant where it is needed, such
as the roots, growing shoots, flowers and fruits. This transport of
food substances in the phloem is called translocation. Water and
mineral ions travel in a different tissue, the xylem, which carries
them up from the roots.
Aphids take in far more sugar than they need, and they release the
extra as a sticky liquid called honeydew. Ants often protect aphids so
that they can feed on the honeydew. Honeydew that falls onto leaves
can grow a black mould, which blocks light from reaching the leaf.
Scientists have used aphids to study phloem. When a feeding aphid is
removed and its mouthparts are cut off, sap continues to flow from the
cut end for hours, and drops can be collected and analysed. Using this
method, scientists measured the concentration of sucrose in the phloem
sap of a plant over 24 hours. The plant was kept in light for the
first 12 hours and in the dark for the next 12 hours. The results are
shown in Figure 1.
Large numbers of aphids weaken plants and can spread viruses from one
plant to another. Gardeners can spray them with an insecticide, but
this also kills useful insects such as bees. Instead, some gardeners
buy ladybirds and release them onto their plants. Both adult ladybirds
and their larvae eat aphids. This is an example of biological control.
Figure 1 Concentration of sucrose in phloem sap over 24 hours
Read the article and study the figure. Use the information in the article and your own knowledge to answer the questions. The lines in brackets show where to look; select them to highlight the text.
Use the information in the article and your own knowledge to answer the questions. The lines in brackets show you where to look; select them to highlight the text.
State what is meant by the term translocation.
(1)Give two differences between the substances transported in the phloem and in the xylem.
(2)Use Figure 1 to calculate the percentage increase in sucrose concentration between 0 and 12 hours.
(2)Explain the change in sucrose concentration shown in Figure 1.
(3)Explain one advantage and one disadvantage of using ladybirds rather than an insecticide to control aphids.
(2)Suggest why large numbers of aphids reduce the growth of a rose bush.
(2)Total for this question: 12 marks
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